In recent years, in light of alternative oil resources and the growing environmental awareness, materials obtained from reproducible natural fibers have been getting attention. Among natural fibers, cellulose fibers having fiber diameters of 10-50 μm and especially cellulose fibers from wood (pulp) have been widely used until now for paper products. In addition, fine cellulose fibers having fiber diameters of 1 μm or less are also known as the cellulose fibers. A sheet containing such fine cellulose fibers is advantageous in that it has high mechanical strength and thus there have been attempts to use the sheet for various applications (Patent Literature 1). For example, it is known that nonwoven fabric obtained by papermaking of fine cellulose fibers is used as a high-strength sheet.
A method for producing fine fibers, which is often used in the art, comprises introducing electrostatic and/or steric functional substituents into a fiber material to facilitate fiber refinement (fibrillating) of the fiber material (e.g., Patent Literature 2 and 3). Patent Literature 2 and 3 disclose that hydrophilic carboxyl groups are introduced onto the cellulose surface such that repulsion between fibers is enhanced, thereby allowing fiber refinement (fibrillating) by mechanical treatment with relatively small energy. However, fine fibers into which such substituents have been introduced tend to experience temporal or thermal discoloration to yellow or brown (hereinafter collectively referred to as “yellowing”), and in the worst case, discoloration to dark brown or black, which has been problematic. In addition, when slurry is prepared using the fine fibers into which substituents have been introduced to produce a sheet by a paper making method or a coating method, freeness is poor, dehydration takes a long time, and productivity extremely decreases, which also has been problematic.